Are sentries in Terraria worth using at all?

on a summoner, yes. sentries use their own slots, not your minion slots, so you're running both at once for free. best on bosses where you're staying in one arena anyway. queen spider staff in early hardmode, the old one's army ones if you've farmed that event, lunar portal staff at the end. on any other class they're pretty weak though, they scale off summon damage.